The term bifocal is used to describe a lens in which the top portion of the lens has the prescription for distance vision, and the bottom portion of the lens has the prescription for reading. The two areas of vision are divided by a visible line in the lens. Bifocals let you focus through two different prescriptions at different distances through the same lens — hence the name bifocals.
Many people need some visual correction in order to read or see things close-up. Often, bifocals are necessary because people's arms "become shorter" as they enter middle age and develop near vision problems associated with presbyopia. However, other conditions can cause people of any age to need more help seeing properly in the near range. Overconvergence, when the eyes work too hard to see close-up, is one.
Regardless of the reason you need a prescription for near-vision correction, bifocals all work in the same way. A small portion of the eyeglass lens is reserved for the near-vision correction. The rest of the lens is usually a distance correction, but sometimes has no correction at all in it.
For golf players, lined bifocal lenses are often fit extremely low and in the inside corner of one lens of a pair of "golf glasses." Just enough near vision correction is placed in the eyeglasses to allow the golfer to read and write on the scorecard without compromising the vision necessary to hit the sweet spot on the ball.